A. The Pathology of Aortic Regurgitation.
Heart valve abnormalities are a major component of cardiovascular disease. Aortic Regurgitation (AR), also known as Aortic Insufficiency (AI), is probably the most common valvular problem. Each year thousands of patients experience cardiovascular function problems as a result of aortic regurgitation. Eventually many of these cases lead to the need for surgical intervention such as aortic valve replacement. Therefore, the detection and evaluation of aortic regurgitation is extremely important in those subjects with suspected cardiovascular disease. The development of a simple, inexpensive technique by which to screen individuals for aortic regurgitation is extremely important in those subjects with suspected cardiovascular disease. The development of a simple, inexpensive technique and apparatus with which one can screen individuals for aortic regurgitation represents an advance in medical instrumentation.
Aortic regurgitation may be caused by a variety of diseases or acute trauma. In the case of disease, the process may act directly on the aortic valve leaflet or the wall of the aortic root. Approximately two-thirds of severe aortic regurgitation cases which result in aortic valve replacement are caused by leaflet abnormalities. As used in this application, the term "aortic valve abnormalities" is broad enough to encompass all of the various conditions which result in aortic regurgitation.
Rheumatic fever is a common disease mechanism of many valve leaflet abnormalities. The fever causes the cusps to become infiltrated with fibrous tissues and retract, a process that prevents the cusps from closing during diastole. This usually results in AR in the left ventricle through the center of the valve. Diseases such as infective endocarditis may cause aortic regurgitation through a similar mechanism.
In contrast, diseases such as Syphilis, Ankylosing Spondylitis, Rheumatoid Arthritis, and Marfan Syndrome may produce aortic regurgitation by causing marked dilation of the ascending aorta. In each of these diseases, the aortic annulus may become greatly dilated, the aortic leaflets may separate, and AR may ensue. In addition, the dilation of the aortic root may have a secondary effect on the aortic valve, since it may cause tension and bowing of the cusps which may thicken, retract, and become too short to close the aortic orifice.
Acute trauma may produce aortic regurgitation as a result of mechanical damage. For example, a tear in the ascending aorta may cause loss of valve leaflet support and therefore lead to the initiation of regurgitation.
Regardless of the etiology, Aortic Regurgitation usually produces dilation and hypertrophy of the left ventricle as a result of the chronic regurgitant flow. It may also produce dilation of the mitral valve ring and the left atrium. These changes represent cardiovascular system adaptation as a result of chronic or gradually increasing aortic regurgitation. The systemic response permits the ventricle to perform as an effective high compliance pump. As a result, patients with severe chronic AR have the largest end-diastolic volumes of those with any form of heart disease.
High end-diastolic and stroke volumes assist in maintaining proper cardiovascular function. As the left ventricle dilates, ventricular function deteriorates due to the inability to efficiently move blood out of the heart. Rising end-diastolic volumes eventually cannot compensate for the regurgitant volume, and the ejection fraction and forward stoke volume decline. In order to restore forward stroke volume and ventricular function, aortic valve replacement usually must be performed.
Unfortunately, the cardiovascular system cannot adapt quickly to acute aortic regurgitation. As a result, the back flow of blood through the damaged valve will fill the ventricle. A ventricle of normal size cannot accommodate the combined large regurgitate volume and atrium inflow. Since total stroke volume cannot rise due to structural constraints, forward stroke volume will decline. In response, left ventricle diastolic pressure may rise quickly, and cardiac function may drop drastically. Cardiovascular complications may ensue quickly threatening the life of the patient.
B. Prior Art Methods for Detecting and Evaluating Aortic Regurgitation.
The state-of-the-art methods for detecting aortic regurgitation and either evaluating the severity of disease or quantifying the amount of regurgitate volume include echocardiography, invasive catheterization, and magnetic resonance imaging (MRI).
A variety of echocardiography techniques can be utilized to evaluate aortic regurgitation. Although M-mode or two-dimensional ultrasound may be quite useful to detect aortic regurgitation or structural changes, the addition of Doppler may be quite useful to measure the outflow velocity from the aortic valve. When combined with measurements of valve diameter, the flow can be calculated. Color flow Doppler represents a drastic improvement in echo imaging due to the ability to approximate the regurgitate volume. Additionally, continuous wave Doppler may also be a useful technique to evaluate the severity of the disease in which the deceleration slope of the ventricular pressure gradient is evaluated. This is accomplished using the Bernoulli equation which relates velocity changes to a pressure gradient.
Invasive techniques may also be used to evaluate aortic regurgitation. Many of these invasive techniques utilize a scale from 1 to 4+ to evaluate the severity of the aortic regurgitation. This is accomplished using angiography techniques to review the degree of regurgitate back flow through the aortic valve.
Recently, however, major advances have been made using MRI to evaluate aortic regurgitation. MRI can be used to simultaneously evaluate the severity of both aortic regurgitation and left ventricle dysfunction. Past MRI techniques utilized multiple tomographic planes which made the process time consuming and difficult to analyze. In addition, the techniques focused simply on the size of the regurgitate flow jet, which has a poor correlation to regurgitate volume. However, recently developed techniques utilize a rapid single-plane cine MRI technique which can be completed in less than 10 minutes. The new technique incorporates a new grading system which is based on the presence, size, and persistence of not only the regurgitate jet, but also the zone of proximal signal loss.
Unfortunately, invasive catheter procedures, echocardiography, and MRI are associated with several problems which may limit routine clinical utilization. Invasive catheterization, for example, is extremely expensive due to the cost of the physician, support personnel, and hospital overhead. These procedures may also be associated with considerable patient risk due to their invasive nature. Additionally, although highly accurate, these procedures are quite time-consuming to perform and usually require an overnight hospital stay. Therefore, few individuals undergo evaluation of aortic regurgitation using invasive techniques.
Non-invasive echocardiography procedures may reduce costs since they can be performed on an out-patient basis, however, they still require the cost of highly skilled personnel. Echocardiography is usually performed by a highly skilled technician and study results are usually evaluated by a specialized physician (cardiologist). Reproducibility may be of concern, however, since results may vary depending on the placement of the non-invasive transducer and the ability of the operator. In addition, the use of two-dimensional imaging may potentially underestimate or overestimate the size of physiological structures since the third dimension in space cannot be evaluated. Potential patient risk may be minimized due to the non-invasive nature of the procedure. However, the time requirements may still potentially limit utilization in some patients.
Although the development of new MRI techniques may represent an advance in the clinical assessment of aortic regurgitation, the expense of such procedures is of great concern. MRI equipment is extremely expensive, and patient access is quite limited. Importantly, although no biological after-effects have been seen from MRI, the body is exposed to low energy radiation which could be potentially hazardous. Further, the operation of an MRI requires highly skilled operators including qualified technicians and a specialized physician (radiologist). Although new methods may reduce procedure time, patient preparation time is still considered very significant.
Room for improvement exists over the known methods for determining aortic regurgitation. In particular, improvement can be achieved by providing a reliable method for determining the existence of aortic regurgitation, and a method for enabling the physician to perform a semi-quantitative analysis of the volume of aortic regurgitation, which does not require an invasive procedure. Further, the state of the known art would be improved by the existence of a method for determining and quantifying aortic regurgitation that can be performed easily by relatively low cost personnel, especially if the method could be performed on the patient without the need for expensive equipment.
It is therefore one object of the present invention to provide a method and apparatus for determining the existence of aortic valve abnormalities of the type that cause aortic regurgitation. It is also an object of the present invention to provide a method and apparatus that can enable a practitioner to determine the existence of aortic regurgitation, and to perform a semi-quantitative analysis of the relative volume of aortic regurgitation.
It is a further object of the present invention to provide a method and apparatus for determining aortic regurgitation which does not require expensive equipment or invasive procedures.